Evaluation of pore structure characteristics of deep clastic rocks in the Huangliu formation of LD-X area, Yinggehai Basin

The LD-X area of the Yinggehai Basin hosts a promising high-temperature and high-pressure tight gas sandstone reservoir with significant resource potential. However, due to the influence of high temperature and carbon dioxide filling, the diagenesis of this reservoir is complex, making the evaluation and characterization of pore structure challenging. Data such as XRD, physical properties, rock slice analysis, electron microscopy, NMR, HPMI, core description, and logging curves were used to systematically analyze the pore type, size, and shape in the LD-X area. The fractal characteristics of small pores, transition pores and large pores are discussed, and the evaluation method of pore structure is established based on the fractal parameters of mercury injection and nuclear magnetic resonance T2 spectrum. The main results are as follows: (1) The pores in LD-X area are mainly intergranular solution pores and feldspar solution pores, with limited retention of primary pores and a considerable reduction in porosity due to calcite cementation; (2) The pore structure within the studied formation is highly complex, resulting in significant variations in mercury influx curves, making the correlation between NMR and mercury injection parameters with porosity and permeability poor; (3) The study area has strong compaction and poor sorting, and due to the existence of multi-stage carbon dioxide filling, calcite cementation is very common and reduces the complexity of pore morphology for small pore. As a result, the larger the fractal dimension of the small pore section, the greater the porosity and permeability; (4) The proposed pore structure evaluation method, integrating the fractal analysis of mercury injection curves and NMR T2 spectrum curves, enables a more precise assessment of pore size, proportion, and shape. It is observed that favorable pore structures frequently correspond to layers with high resistivity and low density in the middle and upper part of the sandstone section on logging profiles. This method provides a new idea for the evaluation of pore structure and favorable reservoir of deep clastic rocks.